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Much has been made of the need for equipment vendors to offer OC-192 (10G bit/sec) interfaces and to be first out of the gate with the next step up in transmission speed, OC-768 (40G bit/sec). And why not? The faster networks are needed to keep up with the growing demand on the Internet.

But very little has been said about polarization mode dispersion (PMD), a problem that could prevent a lot of carriers from upgrading their networks with these new technologies.

Now, however, start-up Yafo Networks reckons it’s close to offering carriers a way around the PMD problem -- with a compensator that will enable them to deploy higher-speed gear on long-distance routes.

Early prototypes of the PMDC-10, as it’s called, are already being evaluated by "four or five" vendors thinking of incorporating it in their equipment, according to Henry H. Yaffe, Yafo Networks’ founder, chairman and CTO. Yaffe says commercial prototypes will ship in a couple of months.

Developing a PMD compensator is a big deal, mainly because PMD is much tougher to control than other forms of dispersion -- due to which signals become increasingly smudged and unreadable as they travel along fiber. This happens because some parts of the light pulse travel slightly faster than other parts.

As the name implies, PMD is caused by light traveling faster in one polarization plane than another. Fundamentally, it happens because the core of the fiber is not perfectly round in cross-section. As a result, the thickness isn’t absolutely identical on every possible axis.

Up until about 5 years ago, manufacturers simply couldn’t produce fiber that was circular enough to keep PMD under control at bandwidths exceeding OC-48 (2.5G bit/sec), according to Yaffe. Fiber happens to have a "sweet spot" at OC-48 that prevents PMD being an issue, he says. At higher speeds, however it becomes increasingly problematic. It’s "pretty bad" at OC-192 and creates a "real mess" of signals at OC-768.

The upshot is that carriers with older fiber in their backbones -- like AT&T, Sprint and WorldCom -- are now lumbered with infrastructure that can’t really support the latest transmission technologies, Yaffe says.

And if they can’t use the already installed fiber, the advancements in transmission speeds are worthless. Enterprise customers will never realize those advanced services that can only be made possible from the deployment of these super-fast pipes.

Other carriers also face problems because otherwise circular cross-section fiber can be squashed out of shape during installation or at any time afterwards, either permanently or temporarily. A lot of fiber is laid alongside railway tracks, for instance, and vibrations from passing trains can flatten it slightly, creating temporary PMD problems, according to Yaffe.

The fact that PMD problems can come and go like this makes it tough to compensate for them. Other dispersion problems don’t change over time and can be dealt with by installing static compensators. PMD compensators, however, have to work dynamically. Yafo's PMDC-10 continuously adjusts the orientation of the light signals and the time difference between the two polarization planess to optimize the signal quality, Yaffe says.

The same concept could be used for automatically adjusting other transmission parameters when traffic is rerouted around a problem in an optical backbone, Yaffe notes. "This is a platform of technologies that lends itself to a whole plethora of applications," Yaffe says.

Downsides? The PMDC-10 occupies a lot of real estate. Each wavelength requires a whole blade of components, and there are 10 blades per shelf and three shelves per bay. In other words, PMD compensation for a 160-channel DWDM system might occupy as many as six bays of equipment. Yafo Networks isn’t talking about price yet, but it’s a fair bet that it won’t come cheap.

At least one other start-up, Phaethon Communications, is thought to be developing a PMD compensator. Phaethon is still in stealth mode, but its Web site says that it’s "devoted to delivering OC-192 and OC-768 data rates over all types of fiber."

This story, "Could Yafo bust network speed limits? " was originally published by
Network World.